When displaying a series of pictures in video format, an aesthetically appealing way of moving from one picture to the next is achieved using “transition effects”. Such transition effects take on a variety of forms and essentially describe any progressive replacement of a first picture by a second picture. A number of different types of transition effects are known, including but not limited to vertical wipes (in which “curtains” open or close from either side of the screen), double wipes (in which the “curtains” open or close from both sides of the screen simultaneously), horizontal wipes, diagonal wipes, squares closing in or out in a checkerboard style, radial wipes and the like.
An understanding of how such transition effects are achieved in the prior art requires a previous understanding of current analogue and digital video formats.
The usual model for analogue video follows the conventions of film based moving pictures, which is a series of still pictures. Digital video maintains this model, but often differs considerably in structure from analogue video. The movement away from the intuitive structure of a series of directly coded independent pictures has been motivated by the huge amounts of information associated with video. Most digital video standards seek to take advantage of spatial and temporal redundancy in the sequence of pictures to achieve significant data compression. There are two approaches to compressing video data, namely lossless and lossy. Lossless compression seeks to store the video content in exact form, whereas lossy compression further reduces the data required by representing only an approximation to the original video. This results in much higher compression and is done in such a way that the visual impact of the compression is minimised.
The widespread and increasing availability of digital video cameras for capturing high quality video, together with the ubiquity of software for viewing digital video and the stability of the content in digital form, makes digital video an attractive medium. However, the amount of data required to store digital video, even in compressed form, is an impediment for some applications. For these applications, it is imperative to use the most efficient forms of video coding, which will inevitably mean some form of lossy compression.
Among the most efficient forms of compression for coding digital video are the standards produced by the Motion Pictures Experts Group (MPEG). However, their efficiency is at the cost of complexity in their internal working. This complexity makes it difficult to manipulate the video in compressed form.
The re-encoding stage requires significant processing capabilities which are often beyond the capabilities of personal computers used in homes and small businesses.
Two of the MPEG standards are colloquially known as MPEG1 (MPEG1) and MPEG2 (MPEG2). These are described in detail in the International Standards Organisation documents which for MPEG1 are numbered ISO/IEC 11172-1 (1993) to ISO/IEC 11172-5 (1998) and for MPEG2 are numbered ISO/IEC 13818-1 (1995)-ISO/IEC 13818-10 (1999). The most important of these documents from the point of view of the embodiments of the current invention are the parts of the respective standards dealing with video coding. These consist of the document ISO/IEC 11172-2 (1993) entitled “Information Technology—Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 Mbit/s: Part 2—Video”, (MPEG1) and the document ISO/IEC 13818-2 (1995) entitled “Information Technology—Generic Coding of Moving Pictures and Associated Audio Information—Part 1 Systems: Part 2—Video” (MPEG2).
These MPEG standards are lossy video digital compression standards that are widely used. The following description relates to these techniques which, for simplicity, will be collectively referred to as MPEG, except where it is necessary to differentiate between the two standards. Much of what is described also relates to the new standard known as MPEG4 and it will be appreciated by those skilled in the art that the present invention may be readily adapted to work with that and other standards.
Because of the complexity of the MPEG format it is difficult to manipulate an MPEG video without first decoding it to a sequence of independently coded pictures. A disadvantage of this decoding step is that it produces a huge increase in the volume of data that has to be handled.
Because the MPEG compression scheme is lossy and not idempotent, a further disadvantage of the decoding and re-encoding step is that it causes degradation of the original content.
The current method of generating an MPEG transition involves decoding the sequence, inserting pictures for the transition and then recoding the sequence. One disadvantage of this method is that it is inefficient. Another disadvantage is that the recoding step degrades the sequence of pictures that are recoded.
A further disadvantage of current methods of generating transitions is that such effects can only be generated by referring to particular video sequences. For this reason, such transitions cannot be re-used unmodified for insertion into any video sequence, but must be individually generated every time a transition is required.
The above discussion of the existing art is intended to assist the addressee in understanding the invention that is described below and should not be taken as an indication of the extent of the common general knowledge of a person skilled in the art.